As is well known in the art, a hydraulic control system of an automatic power transmission uses one or mor shift valves each of which is adapted to control shifting between predetermined two gear ratios when the transmission shift lever is in the automatic forward drive range position. Each shift valve is subjected to a throttle pressure variable with load on the engine and to a governor pressure variable with vehicle speed so that an upshift or a downshift between these two gear ratios is automatically effected depending upon the relationship between the load on the engine and the vehicle speed under part throttle conditions of the engine.
When the accelerator pedal is depressed a distance greater than a predetermined value so as to produce a full throttle condition in the engine or a kickdown condition in the transmission system, another fluid pressure is applied to each of the shift valves in the hydraulic control system so that each shift valve is caused or urged to produce downshifting between the gear ratios allocated to the shift valve. In the conventional shift valve for a hydraulic transmission control system, however, the fluid pressure responsive to the kickdown condition is applied to that pressure acting area of the shift valve which is to be reacted upon by the throttle pressure under part-throttle conditions of the engine. For this reason, the respective shift points for downshifting under part-throttle and kickdown conditions can not be selected independently of each other and as a consequence it has been practically impossible to make available such shift points that are satisfactory for both of these conditions.